Field of the Invention
The present invention generally relates to mobile communication networks—hereinafter, mobile networks. More specifically, the present invention relates to mobile networks based on a “Centralized Radio Access Network” (CRAN) architecture.
The work leading to the present invention has received funding from the European Union's Seventh Framework Program FP7/2007-2013 under grant agreement n. 317941—project iJOIN.
Overview of the Related Art
A CRAN typically comprises one or more central units each one connected to a plurality of remote units (preferably by means of respective optical fiber links).
According to a conventional CRAN architecture, in the downlink, the central unit receives data to be transmitted to user equipments of the mobile network, and provides a digital base-band signal. The digital base-band signal is then converted from electrical to optical, and transmitted to the remote units over optical fiber links. Each remote unit receives the optical base-band signal, which is then converted from optical to electrical, filtered, converted from digital to analog, up-converted from base-band to radio-frequency and amplified, before being radiated by antennas to the user equipments.
In radio access technologies based on OFDMA (“Orthogonal Frequency Division Multiple Access”), such as WiMAX (“Worldwide Interoperability for Microwave Access”), and LTE (“Long Term Evolution”)/LTE-A (“LTE-Advanced”), the digital base-band signal is a composite digital base-band signal, namely it is composed of signals for different user equipment (or user signals) that are superimposed in the time domain.
Each conventional CRAN (based on optical fiber links) thus provides for transmission of one (optical) composite base-band signal (over the fiber links) for each (transmitting) antenna of each remote unit when downlink is considered, or of one (optical) composite base-band signal (over the fiber links) for each (receiving) antenna of each remote unit when uplink is considered, with such a transmission that takes place in the time-domain (e.g., according to “Open Base Station Architecture Initiative” (OBSAI) or “Common Public Radio Interface” (CPRI) standards. As a result of that, as the number of remote units connected to a given central unit increases, the available transmission capacity on the fiber links may rapidly become a bottleneck. Additionally, the available transmission capacity on the fiber links may also be saturated by a relatively low number of signals, especially when taking into account the growing data rate that the new radio access technologies impose—and that is expected to grow further with the forthcoming techniques such as “Multiple Input Multiple Output” (MIMO) and carrier aggregation techniques.
Some prior-art solutions have been addressed to such issues.
Signal processing algorithms have been proposed (such as “adaptive beamforming” and “network coordination” algorithms) that operate on each user signal separately.
WO2006/102919, “A radio Access Method, related base station, mobile radio-network and computer program product using an assignment scheme for antennas' sectors”, discloses a system comprising a radio base station for a mobile network, and a set of remote units connected to the radio base station, preferably via a “Radio Over Fiber” (ROF) arrangement. Each remote unit provides radio coverage by means of a set of communication channels, and is equipped with a respective set of antenna elements. The communication channels are distributed over the antenna elements according to an assignment scheme that is dynamically variable.
WO2010/075864, “A Method for distributed Mobile Communications, corresponding system and computer program product”, discloses a method of arranging exchange of signals between user terminals in a cellular communication system and at least one base station. The base station includes a central unit and a plurality of remote units. The signals are exchanged between the central unit and the remote units as aggregated signals for plural user equipments. The signals are processed at the remote units as distinct signals each associated to a respective one of the plural user equipments. Specifically, WO2010/075864 illustrates a method to reduce data over radio over fiber links (usually referred to as “fronthaul”, in order to distinguish these links from “backhaul” links, which instead connect the central unit to the mobile transport network) based on frequency domain transmission. This is achieved by arranging IFFT/FFT modules in the remote units, and transmitting signals in the frequency-domain.